As seawater desalination apparatuses, as shown in Patent Documents 1, 2, there are known systems for obtaining fresh water by reverse osmotic pressure by using a reverse osmosis membrane (RO membrane).
As such a desalination apparatus, there is known an apparatus having a two-stage structure of a first module and a second module. That is, the first module includes a RO membrane and separates seawater into permeate water and concentrated water by being subjected to reverse osmotic pressure. The permeate water having a salt concentration reduced in this way is further separated into permeate water and concentrated water by the second module having the same structure as the first module, so that the permeate water obtained by the second module is used for drinking or the like.